Propellant extrusion using shaped perforation pins

ABSTRACT

An extruded stick-type propellant grain is disclosed that includes a length separated from a strand of extruded propellant material having a predetermined cross-sectional shape provided with a predetermined number of spaced, radially distributed, hollow non-round longitudinal perforations formed in said length of propellant material parallel to and extending along the length thereof and separated and surrounded by a web of the propellant material. The perforations have a geometric cross section selected from the group consisting of non-round shapes that enhance uniformity in unburned web thickness during a normal burn of said length of extruded propellant thereby reducing the amount of unburned slivers. A central perforation is also provided.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to the field of propellant charges, particularly propellant charges suited for use in large or medium caliber projectile ammunition made up of extruded stick-type propellant grains and to the solution of a shortcoming associated with burning characteristics of extruded stick-type propellant shapes having multiple perforations. Specifically, the use of certain non-round longitudinal perforation shapes in propellant stick grains offers a technique to significantly reduce propellant slivers associated with multi-perf propellant at burnout.

II. Related Art

The success of all ammunition projectiles depends greatly upon the performance and the reproducibility of the performance of the associated propellant system. In this regard, the use of multi-perforated stick propellant shapes have long been utilized to enhance the progressivity of the mass rate of gas generation to enhance ballistic performance of many propellant systems. Control of the burn has been enhanced for certain types of munitions by the use of perforated extruded stick propellant shapes packed into the munition cartridge to be fired.

Almost all extruded gun propellants have perforations parallel to the lengthwise dimension of the extruded stick grains to provide ballistic progressivity as the propellant burns. Depending on size and application, stick propellants are normally processed with 1, 7, 19 or up to 37 perforations to enhance progressivity. Proper progressivity is necessary to achieve the needed performance in modern gun systems. Extrusion tools, which are commonly referred to as extrusion dies, are designed to produce an extrudate having the desired shape including the internal voids associated with the perforations. The physical shape, of course, is determined by the requirements of the gun ammunition system. Extrusion dies of the class described are provided with die pins that are used to impart perforations in the finished propellant grains. Typically, 7-perf gun propellant grains, for example, are provided with one central perforation and a single row radial pattern of six perforations surrounding the central perforation. Other patterns include 19 perforations (2 row pattern) and even 37 (3 row pattern) or more perforations in certain propellant designs. Traditionally, the pins and, consequently, the perforations have been round, owing at least in part to limitations in die manufacturing techniques.

FIGS. 1(a)-1(l) illustrate in cross section progressivity of a burn in a prior art 7-perf propellant grain utilizing round perforations and assuming that the perforations are perfectly round and uniformly distributed within the grain such that, based on the web dimensions illustrated in FIG. 1(a), a=b=c in the grain 10. Note that if uniform enlargement of the perforations and burning from the outside are assumed, this results in the creation of isolated or unburned inner slivers 14 and outer slivers 16. This model mimics a typical burn. The larger outer slivers as at 16 have been a particular problem with regard to ammunition fired with combustible cartridge cases in as much as the existence of any residual burning material in the breach of a cannon after firing such a shell is highly undesirable with respect to the loading and firing of subsequent rounds. The problem has been addressed by providing shaped recesses in the outside of the grains corresponding to the areas of largest sliver formation, however, this has not provided a successful solution.

The need to reduce slivering associated with burning perforated stick propellant grains has remained a longstanding problem in the art and there remains a need for improving the performance of gun propellants by reducing propellant slivers associated with conventional multi-perf propellant geometries.

SUMMARY OF THE INVENTION

The present invention provides an extruded stick-type perforated propellant grain that maintains the ballistic progressivity of earlier grains as the propellant burns yet greatly reduces residual slivers associated with prior stick-type perforated propellant grains. The propellant grain of the invention utilizes perforations having a geometric cross-section of a non-round shape that enhances uniformity in the unburned web thickness during a normal burn. The more uniform reduction of web thicknesses throughout the grain greatly reduces the size of any unburned propellant slivers. One shape that has been found to work successfully in round stick propellant grains has a geometric cross-section resembling a trapezoid radially deployed with the longer base directed toward the outside of the propellant grain. The bases, and particularly the longer base, of the trapezoid is preferably also curved in accordance with the outer shape of the propellant grain itself.

While a trapezoidal shape in the radially spaced array of grain perforations is shown in the detailed embodiment, it will be appreciated that perforations of other geometric shapes may also be used so long as they achieve the desired reduced slivering in progressive propellant grain burns. In accordance with the invention, a central perforation may also be round or another shape such as in the form of a six-pointed star shape in accordance with improved performance during the burn.

Perforations of a generally trapezoidal shape have been found to be optimal with respect to a 7-perf round stick propellant grain such as that illustrated in the detailed embodiment and should also provide improved performance with respect to 19 and even 37-perf grains. The optimal shape of pins for other applications may differ and these may be determined by propellant grain testing.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference characters are utilized to denote like parts throughout the same.

FIGS. 1(a)-1(l) represent a series of enlarged cross-sectional view through a prior art 7-perf stick-type propellant grain illustrating burn progressivity and sliver formation;

FIGS. 2(a)-2(j) depict cross-sectional views of a 7-perf stick propellant grain provided with generally trapezoidal perforations and illustrating a burn progression;

FIG. 3 is a greatly enlarged top perspective view of a die which may be utilized to produce a propellant grain having a cross-section substantially as shown in FIGS. 2(a)-2(i);

FIG. 4 is a bottom perspective view of the extrusion die of FIG. 3; and

FIG. 5 is a greatly enlarged top view of the extrusion die depicted in FIGS. 3 and 4.

DETAILED DESCRIPTION

The process of using certain non-round shaped pins for producing perforations parallel to the length of stick-type propellant grains facilitates the production of stick propellant grains having a progressive burn characterized by a dramatic reduction in the size of propellant slivers resulting at burnout.

In FIGS. 2(a)-2(j) there is shown a series of cross-sectional views of a 7-perf propellant stick 20 having a round central perforation 22 surrounded by six substantially equidistant radially distributed perforations having a generally trapezoidal shape. The trapezoidal shapes have radially directed inner shorter bases 24 and outer longer bases 26. The bases 24 and 26 may preferably be of arcuate shapes generally matching a connecting circle as depicted by broken lines at 28 and 30, respectively (FIGS. 2(c)).

As can be seen in the progressive burn figure representations, the unburned web 32 is consumed in a manner that produces an unburned web residue of a generally uniform thickness that greatly reduces slivering as the progressive burn is completed. This not only reduces problems associated with unburned slivers, but it also enhances propellant performance efficiency by increasing timely consumption.

FIGS. 3-5 illustrate views of a 7-perf extrusion die of a type that can be employed in producing propellant grains in accordance with the present invention. The die shown generally at 40 may be of a single piece design with no pin plate and the pins machined directly into the die. This design enables straight-through extrusion of the propellant with minimal deflection of the shaped die pins. The die includes an entrance end of gradually reducing diameter which determines the desired grain diameter at 44. The pins include a generally round central pin 46 and generally trapezoidal pins 48 radially distributed about the central pin in equidistant fashion. The pins are supported in spaced relation by an integral central web structure in the die 40. The straight through die design facilitates extrusion and minimizes sideways stress on the free ends of the pins as the propellant splits at the web and reforms as it passes through the die. This type of die is more fully described in application Ser. No. 10/698,091, entitled “PIN PLATE-FREE EXTRUSION DIE DESIGN”, filed Oct. 31, 2003, and assigned to the same Assignee as the present invention. It should be noted that the radially distributed pins in the detailed embodiments are depicted as generally trapezoidal in geometry but can be manufactured into other shapes, including triangles, to obtain the progression desired in the propellant stick grain.

This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct new such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different devices and that the various modifications, both as to the equivalent details and operating procedures can be accomplished without departing from the scope of the invention itself. 

1. An extruded stick-type propellant grain comprising: (a) a length separated from a strand of extruded propellant material having a predetermined cross-sectional shape; (b) a predetermined number of spaced, radially distributed, hollow longitudinal perforations formed in said length of propellant material parallel to and extending along the length thereof and separated and surrounded by a web of said propellant material; and (c) wherein said perforations have a geometric cross section selected from the group consisting of non-round shapes that enhance uniformity in unburned web thickness during a normal burn of said length of extruded propellant thereby reducing the amount of unburned slivers.
 2. An extruded stick-type propellant grain as in claim 1 further comprising a central perforation of a generally round geometry.
 3. An extruded stick-type propellant grain as in claim 1 further comprising a central perforation of a generally star-shaped geometry.
 4. An extruded stick-type propellant grain as in claim 1 wherein said perforations have a geometric cross section selected from generally star and trapezoid shapes.
 5. An extruded stick-type propellant grain as in claim 1 wherein said radially distributed perforations have a geometric cross-sectional shape generally in the form of a trapezoid.
 6. An extruded stick-type propellant grain as in claim 2 wherein said radially distributed perforations have a geometric cross-sectional shape generally in the form of a trapezoid.
 7. An extruded stick-type propellant grain as in claim 3 wherein said radially distributed perforations have a geometric cross-sectional shape generally in the form of a trapezoid.
 8. An extruded stick-type propellant grain as in claim 5 wherein at least one base of said trapezoids has a generally arcuate shape.
 9. An extruded stick-type propellant grain as in claim 1 wherein said propellant grain has from 7 to 37 longitudinal perforations.
 10. An extruded stick-type propellant grain as in claim 8 wherein said propellant grain has from 7 to 37 longitudinal perforations.
 11. An extruded stick-type propellant grain as in claim 7 wherein said perforations include an array of six generally trapezoid-shaped perforations surrounding a central star-shaped perforation.
 12. An extruded stick-type propellant comprising: (a) a length of extruded propellant material having a predetermined cross-sectional shape; (b) a predetermined number of spaced, hollow longitudinal voids formed in said length of extruded propellant material parallel to and extending along the length thereof and separated and surrounded by a web of said propellant material; and (c) wherein said voids have a geometric cross section selected from the group consisting of non-round shapes that enhance uniformity in unburned web thickness during a normal burn of a propellant grain made from the extruded propellant material thereby reducing the amount of unburned slivers. 